Ultrasound imaging systems for medical diagnostics scan a subject with ultrasound beams in a sector or rectangular pattern. A transducer emits and focuses the beams along certain linear directions or scan lines. The emitted beam reflects back, in the form of an echo, acoustic discontinuities to the transducer along the same scan lines. The received beam is converted to electrical signals which generate an image on a two-dimensional display representative of a planar cross-section through the subject. The resolution of an ultrasound scanner depends on how well the beams are locally focused along the scan lines.
The transducers of modem ultrasound imaging systems consist of arrays of small rectangular piezoelectric elements to form the focused ultrasound beams. The active transducer area of the portion of the transducer surface which emits and/or receives the sound waves is referred to as the "aperture". The transmit and receive apertures are not necessarily identical.
It is important to focus the beams both in the scanning plane (azimuthal direction) and in the orthogonal (elevational direction or "out of the image plane") plane to provide a well-defined image of the subject. Focusing of the beam at a certain point in the medium is achieved by delaying the waves emitted from or received at various points in the aperture. By pulsing the aperture's elements with predetermined delays relative to a reference "start" time, the transmitted acoustic waves arrive in phase at the focal point in the medium, interacting constructively, to form beams focused along the desired scan lines. Likewise, by delaying the received echoes such that the echoes from each point along the scan lines are in phase, and then adding the delayed echoes, a signal representing mostly the echoes from points along the desired scan line is formed. As a result, the echoes from points away from the scan line arrive with different delays and tend to cancel each other in the summation. The process of forming the focused beams is called beamforming, and therefore, the devices performing this function are called respectively transmit and receive beamformers.
The ratio of the focal depth, defined as the distance from the center of the aperture to the focal point, to the linear dimension of the aperture in a certain direction (e.g., azimuthal or elevational) is called the "f-number". The azimuthal and the elevational "f-numbers" are generally different. The smaller the "f-number" at a certain focal depth, the better the beam focusing at that depth. For a fixed set of delays corresponding to a certain focal depth, the larger the "f-number", the larger the depth of focus, which is defined as the depth range over which the beam stays focused.
Transducers in which the whole array defines a single aperture to generate all beams are called "phased arrays". Transducers in which the aperture is a subset of the whole array, and the aperture's position on the array is shifted in order to generate different beams, are called "linear" or "curvilinear" arrays.
In most current systems the transducer array consists of one row of rectangular elements arranged sequentially along a straight or curved line lying in the image plane. This arrangement is called an 1D array. Each element in the aperture is connected to a transmit beamformer channel and a receive beamformer channel. By appropriately controlling the beamforming delays, the 1D array may be focused in the azimuthal direction at any desired depth. Focusing in the elevational direction is achieved by means of a mechanical lens or by appropriately shaping the elements along the elevational direction. The elevation focal depth, therefore, is fixed. This is a limitation of the current technology, requiring the use of multiple probes, one for each desired elevation focal depth.
In order to extend the depth range over which elevation focus is achieved, arrays are typically designed with relatively large elevational f-number (low element height). This has the disadvantages of weak elevation focusing and reduced aperture area, resulting in decreased sensitivity.
Variable elevation focal depth can be achieved by constructing two-dimensional arrays (2D arrays) of small square elements and connecting them to independent beamformer channels, thus providing electronic focusing in both the azimuthal and elevational directions. This method, however, requires a number of beamformer channels that is prohibitively expensive. For example, if the array had 128 element columns (in the azimuthal direction) and 64 rows (in the elevation direction), and the symmetrical rows were connected together in pairs, 128*32=4096 channels would be needed. Modem high-end imaging systems typically have 128 beamformer channels, and only a few commercial imaging systems are known to have on the order of 200 beamformer channels.
It has been shown that if only focusing, not steering, needs to be achieved in the elevational direction, then the elevational size of the elements may be substantially increased relative to their azimuthal size. This allows a certain aperture height to be obtained with fewer elements, thus reducing the necessary number of channels. Arrays with multiple rows having much fewer rows than columns are referred to as 1.5D arrays. An article published in Ultrasonic Imaging (Vol. 14, pp. 344-353) show that as few as 3 rows of elements provide some elevation focus enhancement, and 7 rows provide most of the enhancement needed. However, the addition of 4 rows leads to a substantial increase (at least doubling) in the number of beamformer channels.
Other embodiments enhance the elevation focusing without applying additional beamforming delays, by using various combinations of depth-variable element height, apodization, multifocus lenses or multifocus element shaping (e.g., U.S. Pat. Nos. 5,349,262 to Grenon and Vogel; 5,492,134 to Souquet; and 5,677,491 to Ishrak et al.). These methods avoid increasing the number of beamformer channels, but achieve a limited enhancement of the elevation focus over depth. In some cases, these methods sacrifice sensitivity by not using some of the center portion of the receive aperture.
Yet another method, known as "synthetic aperture", shown in U.S. Pat. No. 5,186,175 to Hirama et al shares the beamformer channels between rows of transducers, focusing the rows one at a time, storing the signals representing the partially focused beam, and then combining them to obtain the fully focused beam. This method suffers from a reduced frame rate, susceptibility to motion artifacts, and need for additional hardware for storing and processing the partial beam signals.